Antenna systems are used for wireless communications, for radio detection and ranging (RADAR) applications, and for electronic warfare (e.g., jamming). For efficient operation, it is desirable to align an antenna of the system with the most direct electromagnetic propagation path to a target. For example, dish antennas send and receive radiation most efficiently to and from a direction generally aligned with a central axis of the dish, referred to as the boresight of the antenna. Antenna alignment presents many difficulties, particularly when the antenna system and the target are moving relative to one another (e.g., for antennas mounted to vehicles, or for satellite targets), and when the received electromagnetic power fluctuates (e.g., due to environmental changes in the electromagnetic path between the target and the antenna system).
For satellite communication (SATCOM) applications, particularly for antennas mounted to vehicles, a technique known as conical scanning can be used by an antenna system to track its corresponding target satellite. In conical scanning, the antenna of the antenna system continuously scans in a circle around the central path between the antenna and the satellite. The antenna system determines from signal variations that occur as each iteration of the circle is traversed whether the antenna is pointing at the target, and can control the antenna to track movement of the target. A problem with conical scanning is that the antenna is never pointed directly at the target, thus there is always power loss associated with both signal reception and transmission. For widely separated transmit and receive frequencies, e.g., as required by some communications protocols, the power loss in the transmit band or the receive band can be significant. As the apparent angular velocity of the target increases relative to the antenna beam width, the required scanning speed increases. This can occur for example due to the velocity of the target, rapid movement of the antenna platform, or for an antenna with a very narrow beamwidth. Furthermore, the angle of the alignment error which can be corrected in conical scanning may not be sufficient for large alignment errors, and antenna systems may need to enter an inefficient time-consuming search mode to locate the target.
An alternative technique for target tracking is monopulse tracking, where the antenna system receives propagating electromagnetic energy from two different beam patterns: a central main beam pattern (corresponding to the symmetric “sum” beam) carries energy received along the antenna boresight (i.e., the optical axis or the direction of maximum gain of a directional antenna), while one or more non-central “null” beam patterns (corresponding to “difference” beams) carry radiation received from off-boresight directions that are generally anti-symmetric with respect to the boresight (and therefore the difference beams have a null aligned with the antenna boresight). Asymmetry in the received power between opposite sides of the boresight indicates that the target is off axis. The magnitude and phase of the signals received in the difference beams, relative to the signal received by the sum beam, indicate both the magnitude and direction of the angular alignment error between the antenna boresight and the target, and can be used by a monopulse scanning antenna system to correct its direction and track the target.
However, monopulse tracking generally requires antenna systems with relatively large waveguide feeds to extract the tracking signals (some require large mechanical structures to separately detect the on-boresight and the off-boresight signals), and thus may not be used in at least some tracking applications which require a smaller antenna system (e.g., for antenna systems mounted in vehicles). Furthermore monopulse tracking systems may only provide tracking information in one dimension (e.g., up and down), or may be insufficiently flexible to track a target using received radiation in a variety of polarisations. For example, existing multihole tracking mode couplers can be far too long for certain applications, e.g., in SATCOM for moving antenna systems.
Existing single slot monopulse couplers can be very short, but have discontinuities in the main circular waveguide path which render them unsuitable for applications where there are two widely separated transmit and receive bands because the discontinuities introduce unacceptable levels of higher-order waveguide modes (e.g., TM11 modes in circular waveguides).
It is desired to address or ameliorate one or more disadvantages or limitations associated with the prior art, or to at least provide a useful alternative.